Rotted wood arising from an engineering flaw caused the collapse of a footbridge north of Westchester Lagoon this summer, and an analysis of four other bridges on Anchorage's Tony Knowles Coastal Trail revealed similar signs of decay, according to two reports released this week by the Anchorage Department of Parks and Recreation.

The reports by the engineering design firm Stantec (formerly USKH Inc.) offer the most detailed explanation to date of the collapse of the North Lagoon Bridge, as well as a grim assessment of the state of the other four. None of the bridges is considered an imminent threat to pedestrians. But the parks department says the extent of the decay is such that vehicles are no longer allowed on the bridges, raising challenges for winter maintenance.

More broadly, the reports shed light on a looming multimillion-dollar Anchorage infrastructure problem. Parks officials, previously focused on a trail repaving project, have swung their attention to more than 30 footbridges along the trail system.

The general conclusion is this: Anchorage's greenbelt bridges are rotting away. The design flaw in some bridges, dating back to the 1980s, is allowing water to seep into the wood.

There is no long-term replacement strategy. And if a bridge falls, there is no money to fix it.

Since the Westchester Lagoon bridge collapsed, the parks department and the nonprofit that supports the trail system, the Anchorage Parks Foundation, have pieced together $3 million in bond and state grant funding requests. If all the money comes through, the fallen bridge will be replaced next summer, followed by replacement of a wooden bridge at the north end of the Coastal Trail near Second Avenue downtown, which engineers say also has advanced decay.

Officials say those upgrades would only be the beginning. And a strategy for fixing the rest needs to materialize soon.

"We need to have a plan for how we're going to address the infrastructure over the next 10 to 15 years," parks superintendent Holly Spoth-Torres said in an interview this week.

"If you're thinking about a comprehensive maintenance plan, you don't want to wait until something fails."

A bridge collapses

On June 16, a 2006 Ford F-550 truck was driven onto the 70-foot bridge tucked into the northwest corner of the Westchester Lagoon park, pulling a wood chipper.

The truck weighed about 7,100 pounds. The chipper weighed about 7,300 pounds. They were too much for the the bridge. One beam split, and the fracture spread "like a zipper," the report said. No one was injured, but the bridge, built around 1987, was a total loss.

Later that day, engineers with Stantec drove to the site to begin an emergency evaluation. They examined the wood and found moist, brown surfaces, stains from moisture that over three decades had seeped through openings around bolts.

The bolts apparently were not installed in accordance with modern building codes.

"Although this was not a recommended (design), it did not fail until years of water infiltration and decay weakened" the wood to the point of collapse, the Stantec report states.

The report also concludes: "Had that connection zone not been decayed, the truck and chipper would not have caused failure."

Spoth-Torres said the parks department has tried to research how the bridges were originally designed. It's still not clear why those decisions were made, she said.

"But what we do know is, we would never do that now. Or even 10 years ago, 15 years ago," she said.

Wooden bridges have an aesthetic edge in Anchorage, Spoth-Torres said. People love how they look.

But she acknowledged that in a coastal winter environment, materials like steel are far more durable.

Decay in other bridges

Between June 17 and 19, the engineers analyzed four other Coastal Trail bridges: the Second Avenue bridge, the Fish Creek bridge, the south Westchester Lagoon boardwalk and the Sisson Loop bridge at the edge of Kincaid Park. They spent three days taking pictures and checking stability.

They found that the bridges were built the way the north Westchester Lagoon bridge was -- and had the same issues with decay.

Of all the bridges analyzed, the Second Avenue bridge was found to have the most advanced decay and is the highest priority for replacement, according to the report. A sample taken from the wood was "very wet," crumbling when removed from the girder.

Based on the assessments, all the bridges are safe for large groups of pedestrians, including runners during events like the Big Wild Life Runs. Snowmachines and all-terrain vehicles being used for city maintenance can safely be driven across them.

But pickups and heavy machinery have been deemed too risky. That poses a problem for winter maintenance on some parts of the Coastal Trail. Near the Cook Inlet-exposed Fish Creek Bridge, where coastal weather causes icy snow drifting, the parks department had enlisted the Nordic Skiing Association's heavy Pisten Bully to groom the trail. The city is now trying to figure out a different way to maintain that part of the trail, Spoth-Torres said.

Aging infrastructure

Many of the city's greenbelt bridges date back to the 1980s. The parks department has since completed basic maintenance but no major maintenance, Spoth-Torres said.

Even before the collapse of the north Westchester Lagoon bridge, the parks department knew a major investment was needed in its greenbelt bridges. But other projects that seemed to have more immediate benefit were taking precedence. In 2013, the parks department began a three-year asphalt replacement project. The plan was to tackle the Coastal Trail, Chester Creek Trail and Campbell Creek Trail in consecutive summers.

The department was planning to move to bridges after that, Spoth-Torres said. Then one of those bridges collapsed.

"We weren't expecting this significant of a failure so soon," Spoth-Torres said.

Meanwhile, the design flaw revealed through the engineering analysis is believed to be present in bridges along the older part of the Campbell Creek trail, she said. It's not yet known whether Chester Creek trail bridges contain the flaw.

For now, the department has suspended plans to begin surface rehabilitation on the Campbell Creek Trail in summer 2016 in favor of first focusing on bridge replacements.

Now the big question mark is money.

Expensive job

Anchorage's parks department paid Stantec $20,000 to complete the two studies on the Coastal Trail bridges. With those reports completed, engineers are moving forward with design plans for new bridges.

Spoth-Torres said the parks department is exploring ways to keep costs low and does not yet have final estimates on replacement costs, including for the wrecked north lagoon bridge.

Temporarily reinforcing bridges with steel, meanwhile, would cost between $200,000 and $300,000 each, according to the Stantec report. For a fix lasting only a couple years, that investment isn't worthwhile, Spoth-Torres said.

"I think we just need to replace the aging structures," she said.

That's a big fiscal order, however. Right now, a request for $1.5 million is in the draft municipal bond package to be approved by voters in the April city election. Another $1.5 million is being requested from the Alaska Legislature in the form of a grant.

It's not clear how many bridge replacements those funds could support, Spoth-Torres said. The cost of replacing each of the 37 bridges in the system, meanwhile, has been pegged at tens of millions of dollars at most, though an in-depth analysis has been conducted only on the Coastal Trail bridges.

Anchorage Assembly member Ernie Hall, who serves on the Parks Foundation board, said replacing the Westchester Lagoon bridge is "imperative." But after that, he said, it's hard to say where bridge replacements will fall in the list of funding priorities for the city and state.

When the north Lagoon bridge fell, said Beth Nordlund, executive director of the Parks Foundation, she initially took it in stride. But she's come to realize the magnitude of the problems and the lack of a long-term plan cannot be ignored.

She recalled the unveiling of a public art installation two months ago in Westchester Lagoon. People were taking pictures and talking to the artist, right in front of the broken bridge.

More than once, Nordlund said, someone yelled, "Fix the bridge!"

Nordlund wants to do that, and soon. Her focus now, she said, is working on funding strategies and talking to local and state elected officials to gain political support.

She also noted that time is not on her side.

"I don't have 37 years of my life to give to this," Nordlund said. "All of a sudden, the scope of this thing ... One bridge a year is not gonna satisfy me."